This invention relates to improved ceramic matrix composites. Such composites have been used in applications requiring stability to high temperatures and thermal shock. One example of such an application is the use of ceramic matrix exhaust components in high performance jet engines.
Even though ceramic matrix composite articles are capable of withstanding higher use temperatures than metals there continues to be a need for composites which will withstand repeated exposure to higher temperatures and more severe thermal shocks One of the major causes of failure of composite structures in severe environments is oxidation. Another problem is degradation of properties due to repeated exposure to high humidity between thermal excursions.
There have been a number of attempts to solve these problems. One approach has been to select materials for the composite that inherently resist oxidation. Substitution of silicon carbide fibers for carbon fibers is one example of this approach. However, the more oxidation-resistant materials may not possess the same level of strength as the more readily oxidizeable materials. Further, even if oxidizeable carbon fibers are replaced by more oxidation-resistant fibers, most composites employ an interface debonding layer which generally is pyrolytic carbon or some other material also subject to oxidation. Further, differences in coefficients of thermal expansion between the fibers and the matrix materials, or between the matrix materials and the outer coating can result in cracking, oxidation and failure. The problems caused by differences in coefficients of thermal expansion between substrates and coatings and between fibers and matrices have caused many to focus efforts on composites where the fiber and the matrix are of the same material.
Another approach to overcoming the oxidation of composites has been the use of glass-formers as oxidation inhibitors. These materials have been used as layers over the matrix, and as additives to the matrix itself. Materials such as boron, when oxidized, form a sealing glass which fills passageways through which oxygen enters the composite and prevents further oxidation. A number of patents teach the use of glass formers as oxidation inhibitors in carbon-carbon composites. See U.S. Pat. No. 4,795,677; U.S. Pat No. 4,894,286; U.S. Pat. No. 4,892,790; and U.S. Pat. No. 4,599,256.
In spite of the advances that have been made, there remains a demand for composite materials that are capable of withstanding high temperatures and large thermal shocks as well as exposure to high humidity. This invention provides composite materials which are both easily made and which exhibit greater resistance to oxidation than composites known heretofore. This resistance to oxidation can be demonstrated in conditions of repeated high temperature excursions, high humidity conditions and in high temperature low pressure conditions.